Polyoxymethylene is a well known engineering resin that is produced by polymerizing formaldehyde or trioxane (which is a cyclic oligomer of formaldehyde) or by copolymerizing trioxane with a comonomer, such as a cyclic ether or cyclic formal. The terminal end groups of the polyoxymethylene chain are typically stabilized (e.g., via hydrolysis) to inhibit depolymerization of the polyoxymethylene chain. Antioxidants and other thermal stabilizers have been blended with polyoxymethylene molding compositions so as to further improve their thermal stability characteristics.
Sterically hindered phenol compounds as well as sterically hindered amine compounds are well known antioxidants that have been incorporated into polyoxymethylene molding compositions in the past. In addition, other thermal stabilizers that have been blended with polyoxymethylene resins include polyamides, urea derivatives, amidine compounds, alkali metal hydroxides, alkaline earth metal hydroxides, and organic and inorganic acid salts.
However, polyoxymethylene resin compositions which include such known antioxidants and/or thermal stabilizers may still undergo some depolymerization during molding (i.e., due to the elevated temperatures that are employed during the molding cycle) and thereby emit free formaldehyde as a by-product of such depolymerization activity. Release of free formaldehyde into the ambient workplace is quite noxious and therefore should be avoided. However, as mentioned above, even with known antioxidants and/or stabilizers, some release of free formaldehyde is inevitable and therefore has typically been tolerated in the past with suitable protective measures being taken.
Another disadvantage associated with conventional antioxidants and/or thermal stabilizers is that deposits (e.g., powdery or tarry residues) typically form on the mold walls, particularly after molding has progressed continuously for a significant length of time. These mold deposits can, and often do, build up over time to such an extent that the appearance of the resulting molded part is adversely affected.
It is towards providing solutions to the above-noted problems that the present invention is directed. Broadly, the present invention includes the incorporation of a specified amine polymer (to be described in greater detail below) and an antioxidant into a polyoxymethylene base resin.
Preferred embodiments of this invention will include (based on the weight of the polyoxymethylene base resin) between 0.01 to 5% by weight of a hindered phenolic antioxidant, and between 0.01 to 5% by weight of an amine polymer having a molecular weight of at least 1000 and comprised essentially of structural units of the following formula (1): ##STR2## wherein R represents a hydrogen atom, an alkyl group, a phenyl group, an alkyl-substituted phenyl group, an NH.sub.2 group directly bonded to the carbon atom, or --X--NH.sub.2, wherein X represents an alkylene group, a carbonyl group, a carboxyalkylene group, an alkyleneoxycarbonylalkyl group, a phenylene group, an alkyl-substituted phenylene group, an amido group or an alkyleneamido group.
The polyoxymethylene molding compositions according to the present invention will exhibit improved stability during molding and thereby significantly reduce (if not essentially eliminate) chain depolymerization and the resulting release of free formaldehyde into the ambient environment. In addition, the polyoxymethylene molding compositions of the present invention exhibit improved mold release properties so as to minimize the deleterious effects of mold deposits associated with conventional polyoxymethylene molding compositions.
Further aspects and advantages of the present invention will become more clear from the following detailed description of the preferred exemplary embodiments thereof which follow.